In recent years, in various applications, an optical filter or film through which light in the visible wavelength region (420 to 630 nm) is transmitted, but which blocks out light in the near infrared wavelength region (700 to 1,200 nm) has been used.
For example, in an imaging device such as a digital still camera or a digital video camera employing a solid-stage imaging element (such as a CCD or a CMOS) or a display device such as an automatic exposure meter employing a light receiving element, in order to bring the sensitivity of the solid-state imaging element or the light receiving element closer to the visual sensitivity of the human, such an optical filter is disposed between an imaging lens and the solid-state imaging element or the light receiving element. Further, in a PDP (plasma display panel), an optical filter is disposed on the front side (the viewer's side) to prevent the malfunction of remote control devices for home electric appliances which are operated by near infrared rays.
As the optical filter or film, a glass filter comprising fluorophosphate glass or phosphate glass which contains CuO or the like, so as to selectively absorb light in the near infrared wavelength region, has been known. Further, a reflection type interference filter comprising, for example, a silicon oxide (SiO2) layer and a titanium oxide (TiO2) layer alternately laminated on a substrate, which reflects and blocks out light in the near infrared wavelength region by interference of light, or a film containing a dye which absorbs light in the near infrared wavelength region incorporated in a transparent resin, has been known (Patent Documents 1 and 2).
However, a light absorbing type glass filter is expensive and is hardly made thin, and it cannot satisfactory meet the demands for downsizing and reduction in thickness of imaging devices in recent years. With a reflection type filter having a light reflecting layer formed thereon, the shielding properties vary depending upon the angle of incidence of light, and the color properties are different between the center portion and the peripheral portion thereof of an image. Further, a ghost is likely to occur by entrance of a reflected light as a stray light into the solid-state imaging element.
A film containing a dye which absorbs light in the near infrared wavelength region incorporated is easily processed and is capable of downsizing and reduction in thickness, and is free from problems as in the reflection type filter, such as the angle of incidence. In addition, the film can be directly bonded to the solid-state imaging element or the like due to its form, and it is advantageous in that further downsizing and reduction in thickness of a device can be achieved. However, this film is not satisfactory in the near infrared shielding properties.
Further, a filter comprising a layer made of a resin which absorbs near infrared rays and a layer which reflects near infrared rays laminated has also been developed (Patent Documents 3 and 4). However, this filter is also not satisfactory in the near infrared shielding properties, and it cannot satisfactory meet the demands for downsizing and reduction in thickness.
On the other hand, it has been proposed to impart a near infrared cut filter function to the solid-state imaging element or the lens itself without using the above-mentioned optical filter or film. For example, a solid-state imaging device comprising a layer containing a compound which has an effect to absorb light having a wavelength in the near infrared region, such as an anthraquinone compound, a lens comprising a dielectric multilayer film having an effect to block out light having a wavelength in the near infrared region on its surface, and a lens formed by using a glass material which selectively absorbs light having a wavelength in the near infrared region, have been developed (Patent Documents 5 to 7).
However, with respect to the solid-state imaging device, the near infrared absorbing compound used has no sufficient effect to absorb light having a wavelength in the near infrared region and in addition, the change in the transmittance in wavelengths from 630 to 700 nm is not steep. Further, there are restrictions on the layer formation such that the function as a solid-state imaging device will not be impaired, it has been difficult to impart a sufficient near infrared cut filter function.
Further, with respect to the lens comprising a dielectric multilayer film, as the dielectric multilayer film has a reflection type interference filter function, that is, it reflects and blocks out light in the near infrared region by interference of light, the shielding properties vary depending upon the angle of incidence of light, and the color properties are different between the center portion and the peripheral portion thereof of the image. Further, a ghost is likely to occur by entrance of a reflected light as a stray light into the solid-state imaging device. With respect to the glass lens, the material itself is expensive and in addition, the production cost is also high since a glass material having a high softening point is press-formed. Further, a near infrared cut filter for an imaging device is required to have an effect to block out light having a wavelength in the near infrared region and to have such a property that the transmittance steeply changes in wavelengths from 630 to 700 nm, to obtain a brighter image of a dark portion. However, the above lenses are not satisfactory in these properties.